Infrared detectors are well known in the electronics art and are required for a variety of applications. These applications include non-contact temperature measurement, in-situ monitoring of semiconductor process steps, infrared spectroscopy, detection of various gases and liquids in industrial process environments, applications in medicine such as medical thermography, and for non-contact thermal imaging of wafers and devices in manufacturing environments.
There are generally two types of infrared detectors--photonic and thermal detectors. In photonic detectors, the radiation is absorbed by the material in the detector resulting in a direct modification of the material's electrical properties. In thermal detectors, radiation is absorbed by the thermal material, causing heating of the lattice of the material. The change in lattice temperature is then converted into change in the electrical properties of the structure. While the two-step transduction process of thermal detectors is generally slower than the single-step process associated with photonic devices, in applications where high speed is not of primary importance, thermal detectors have a number of advantages. These advantages include broad spectral response, low cost, ease of operation, and insensitivity to ambient atmosphere. For applications in gas analyzers, intrusion alarms, non-contact temperature measurement, spectrometers, and other applications, these advantages can be significant.
There are two common types of thermal detectors, known as pyroelectric and thermocouple detectors. In pyroelectric detectors, the temperature change alters the dipole moment of a pyroelectric material, resulting in a charge difference between the crystal faces of the material. A thermocouple consists of a pair of dissimilar conductors so joined at two points that an electromotive force is developed by the thermoelectric effect when the junctions are at different temperatures. A thermocouple requires no bias supply, is useful over a wide range of ambient temperatures, and is easily interfaced with monolithic integrated circuits. To increase the output voltage of a thermocouple-type infrared detector, several thermocouples can be connected in series to form a thermopile.
In the past, thermopile infrared detectors have used vacuum evaporation and shadow masking of the thermocouple materials on thin plastic or alumina substrates, as shown in U.S. Pat. Nos. 4,456,919 and 4,795,498. This approach has resulted in relatively large feature and die sizes and in structures which lack the batch fabrication, process flexibility, and on-chip circuit compatibility characteristic of devices based on the full range of silicon integrated circuit technology.
More recently, the application of silicon semiconductor technology to thermopile detectors has also been implemented. The first silicon thermopile detectors were based on thermocouples of various materials and were shown to be capable of relatively high sensitivity, low cost, room temperature operation, and very wide spectral band response. These detectors often included signal processing circuitry on the same semiconductor substrate as the thermocouples. However, the manufacturing process for these devices required alignment of patterns on both sides of the silicon wafer which increased the die area and the cost of the device. In addition, the structure of the thermocouples in existing silicon thermopile detectors was highly inefficient and, therefore, increased the die area and cost of the device. Finally, existing silicon thermopile detectors have been formed using only a limited number of materials for the thermocouple, which greatly decreases the available materials for the devices and can limit the responsiveness of the device.
Silicon thermopile detectors were considered by Applicant Wise in a 1982 paper entitled "A Batch-Fabricated Silicon Thermopile Infrared Detector", 29 IEEE Transactions on Electron Devices 14 (January 1982). A silicon thermopile infrared detector is disclosed, consisting of a series of thermocouples whose hot junctions are supported by a thin silicon membrane and whose cold junctions are formed on a thick supporting rim. The thin silicon membrane was formed by highly doping the intended membrane area with boron. The desired patterns were then defined on the front side of the wafer, for p-diffusion or alignment marks, and on the back side for alignment marks or the membrane opening. The back side pattern had to be aligned with both the crystal axes of the wafer and with the eventual thermopile structure on the front side. Accordingly, the need for front-to-back alignment increased the die area and the device cost. The 1982 publication described thermocouples which were fabricated of interleaved polysilicon and gold layers. Interleaving of the layers increased device size, and the materials involved limited the responsiveness of the device.
Applicant Wise also described silicon thermopile detectors in a 1985 paper entitled "A Linear Thermopile Infrared Detector Array With On-Chip Multiplexing" which appeared in Proceedings of the International Conference on Solid-State Sensors and Actuators-Transducers, Philadelphia, Pa., Jun. 11-14, 1985. Disclosed is a silicon thermopile detector having a thick boron doped rim in a silicon substrate, which functioned as a heat sink to sustain the cold junctions and give mechanical support to the device. In order to fabricate the device, desired patterns were formed on both the front and back sides of the wafer. The thermocouples were formed of interleaved polysilicon and gold layers on a dielectric layer spanning the rim. The back side of the wafer was etched leaving both the boron-doped areas of the wafer and the undoped areas of the wafer. Again, front-to-back etching alignment was necessary, the interleaved layers increased the detector size, and the thermocouple materials limited responsiveness.
Applicant Wise further considered silicon thermopile detectors in another paper entitled "A Silicon-Thermopile-Based Infrared Sensing Array for Use in Automated Manufacturing" which appeared in 33 IEEE Transactions on Electron Devices 72 (January 1986). In that paper Applicant Wise discussed a detector similar to the 1985 detector discussed above. Front and back patterning and alignment was still necessary, and the resulting substrate contained both doped and undoped portions. The thermocouples of these devices were formed of interleaved layers of polycrystalline silicon and gold, thereby increasing device area and reducing sensitivity.
Consequently, even in view of the most recent thermopile detectors, there is still a great need for a thermopile detector which does not require alignment of patterns on both sides of the semiconductor substrate, which has an efficient thermocouple layout, and which can incorporate new materials for the thermocouples to increase sensitivity.